Step 1: Prepare the lava lamp bottle. The first step in building your homemade lava lamp is to find a tall, thin, clear bottle. Any size or shape will do. A plastic bottle, such as a recycled soda or mineral water bottle, will eliminate worries about breakage if it falls. We used a 16-ounce bottle, which is large enough to get the full floating blob effect, but only requires one 12-ounce bottle of baby oil. A larger lamp bottle will last longer but you’ll need more baby oil to fill it. Rinse the bottle clean and remove any labels. A glue solvent like “Goo Gone” can make scraping off a tough label easier.

Step 2: Prepare the Bottom Layer of Colored Water.
Once you’ve got your bottle ready you can begin filling it. The exact chemical formula used to make real lava lamps is a trade secret (although the UK manufacturer Mathmos has a cool video on its website showing the lamp being assembled). But as with the egg timer which inspired it, the gooey “lava” is actually a type of wax. And the way it slowly floats up and drifts down has to do with the balance between the density of the wax and the density of the liquid it floats in.

Density tells us how much of a substance is contained within a certain volume, or amount of three-dimensional space. The higher the density, the more stuff is in there. And density can change with temperature. For instance, at room temperature the wax in a lava lamp is denser than the liquid, so it sits at the bottom of the lamp. When the light is turned on and the wax warms up, it begins to melt and spread. As the volume of wax in the lamp grows, its density decreases, because the same number of molecules are now taking up more space. At the point where the wax becomes less dense than the liquid, it starts to float to the top of the lamp. Eventually the wax at the top, away from the hot light, starts to cool. As it becomes denser, the wax sinks down, is warmed by the light, and the cycle repeats itself in a colorful mesmerizing display.

Our homemade version uses water and oil, but the principle is the same. Oil is less dense than water, so the water in our lamp sinks to the bottom, while the oil floats to the top. When we introduce a less dense gas into the water, bubbles form that carry some of the water slowly up through the oil. By coloring the water with dye, we can approximate a lava lamp-type effect that lasts several minutes.

If spilling food coloring or oil is a concern, place your bottle on a plastic plate or other protected surface. (They can also stain clothing, so old clothes or an apron may be advisable.) Fill the bottle about one quarter full with water. Add 3 to 4 drops of liquid food coloring for a 16-ounce bottle, or more for a larger bottle.

Before you shake up the bottle to mix the food coloring, take a few moments to watch as the dye begin to spread in vibrant tendrils of color, all on its own. That seemingly spontaneous, random movement is called Brownian motion, after the botanist Robert Brown. In 1827, Brown looked at grains of plant pollen in a drop of water through a microscope and noticed that they jiggled around. In 1905, Einstein suggested that the pollen grains were moving because they were colliding with molecules of water, and predicted that the way the grains moved could be used to figure out how many molecules of water there were. Scientists later showed that Einstein’s prediction was correct -- one of the first proofs of the existence of atoms and molecules.

Once you’ve mixed up the food coloring and water as evenly as possible, by shaking, stirring, or just letting Brownian motion do its thing, check to see if you need to add any more dye. To get the lava lamp effect to work, you want the water to look pretty dark. But don’t go overboard at this point. If necessary, you can add more food coloring later on in the process.

Step 2: Add the Top Layer of Oil.
Now that you’ve got your colored “lava” layer ready, it’s time to add the clear liquid. We use baby oil, which is colorless and available pretty much anywhere. Baby oil is mostly mineral oil -- a non-toxic byproduct of the manufacture of gasoline, used to lubricate cooking tools and to make gummy candy -- with a little fragrance added to make it smell nice. Compared to cooking oil, baby oil is also inexpensive. I picked up a 12-ounce-size bottle, enough for one 16-ounce homemade lava lamp, at my local dollar store. Of course, you can use any kind of cooking or skin care oil you have on hand, as long as it’s not too dark to see through.

Pour the oil into the lava lamp bottle slowly, stopping when you reach the bottle’s shoulder. You’ll want to leave a little head room so the lamp solution doesn’t bubble over. Let the bottle sit for a minute or two, so that the contents settle into two layers: colored water on the bottom, oil on top, with a nice sharp line in between.

Here’s where density comes in. As already mentioned, oil is less dense than water, so the same volume of oil weighs less than an equal volume of water. In scientific terms, the specific gravity of mineral oil – or the ratio of its density compared to the density of water – is about 0.8 to 1. However, there’s another reason that oil and water form two neat layers, and it has to do with the bonds that make molecules stick together with other molecules.

If you could see a water molecule, you’d notice two little hydrogen atoms sitting on top of a bigger oxygen atom like the ears on Mickey Mouse’s head. Those “ears,” which each have negatively-charged electrons whirring around them, give that end of the water molecule a slightly negative charge. That makes water a type of polar molecule. Just like magnets, the negative end of polar molecules is attracted to the positive end of other polar molecules. Oil molecules, on the other hand, are non-polar. They’re made up of long chains of carbon and hydrogen atoms, which don’t have a charged side. Non-polar molecules will form bonds with other kinds of non-polar molecules (“like dissolves like”), but when put together with polar molecules, they keep to themselves. So when people say “oil and water don’t mix,” what they’re really talking about is molecular polarity!

Step 3: Set up the lights.
So far you’ve built what science teachers call a density column – one type of substance floating on another. To turn it into a lamp, you need to add a base with a light source. A small battery-powered LED push light is ideal. These look like discs with three or four LED bulbs embedded in them. You can find inexpensive versions in the flashlight section of your local dollar store or discount mart. Just set the LED push light on your plate or protected surface, and carefully place your lava lamp bottle on top so that the liquid inside is lit up.

If you can’t find one of these handy lights, you’ll have to make your own base. A large sturdy plastic jar or a can should work fine. Arrange some new or re-used LED bulbs inside. To power them, slip a button battery between the wires of each LED bulb and tape in place.

Step 4: Start the action!
Unlike real lava lamps, which run on the heat of a light bulb, our homemade lava lamp is powered by the energy released when you drop an effervescent (fizzing) antacid tablet like Alka-Seltzer into water. You’ll probably have to break the tablet in half to fit it in the opening of the bottle, but try to use as large a piece as possible. The bigger the piece, the more dramatic the effect!

As the tablet hits the layer of water at the bottom of the homemade lava lamp, it begins to release bubbles of gas. Because gases have a lower density than liquids, the bubbles float slowly upward through the thick layer of oil, carrying drops and blobs of water along with them. But when the bubbles reach the surface, the gas continues rising, while the denser water that’s left behind drifts back down to the bottom of the lamp. The lava lamp effect will continue as long as the tablets are fizzing. You can keep adding pieces of antacid tablets to prolong the show, until the oil gets too cloudy to see through.

What’s happening when the tablet begins to fizz is actually the same chemical reaction as the classic baking soda-and-vinegar volcano. Fizzing tablets contain an acid (powdered citric acid, which gives lemon juice its sour taste) and a base (sodium bicarbonate, our old friend baking soda). Acids are compounds with an excess of hydrogen (H+) ions, or hydrogen atoms that are missing their electron and have a positive charge. Bases have a surplus of hydroxide (OH-) ions, which are negatively-charged molecules of oxygen and hydrogen. Strong acids and bases are highly reactive – they’ll combine with, and start to dissolve, many types of material, including your clothing and skin. But put them together in the right amounts and they’ll neutralize each other to produce water! (H+ and OH- make H2O.) When fizzing tablets react, they also produce a salt (in this case, sodium citrate) and carbon dioxide (the gassy bubbles).

Know what else is cool? The reaction of fizzing tablets and water creates an endothermic reaction. That means that those chemical changes are pulling heat out of the surrounding water to use as fuel. So as it’s fizzing, your lamp is dropping in temperature. Try sticking a cooking thermometer in your bottle to see if you can detect the change in temperature. (You might want to skip the lights, since they may give off a small amount of heat energy.)

Disclaimer

This blog takes no responsibility for readers who try any of the experiments and demonstrations mentioned herein. Use appropriate safety gear (goggles, gloves, shoes, etc.) and educate yourself about laboratory safety rules.